hp envy 17 intel i5 runs slow

I have an HP envy 17 with an Intel i5 processor and 8 GB of memory. My processor seems to run at a very slow speed ( around 75 MHz) for a computer that should be running at 2500 Mhz. The sound becomes distorted and the computer is almost unusuable. I have turned off anti-virus software and have the same problems. Does anyone else have this type of problem with this machine? Thanks.